Sion's W passive is a nerf, not an ability
Sion's health at level 18 is only 1784. That's on par with the lowest of mages.
Now, Soul Furnace, assuming his cs and takedowns are average according to champion.gg, only gives him an addition 524 by the end of a game. That's still lower than the level 18 base health of most tanks.
So rather than extra health, Soul Furnace does nothing but give the enemies the opportunity to prevent Sion from having average health for his class. This is just unacceptable, really.